1. Field of the Invention
This invention pertains to clamping mechanisms, and more particularly to apparatus for releasably gripping cylindrical workpieces.
2. Description of the Prior Art
Various equipment has been developed to manufacture and resharpen cutting tools such as twist drills and end mills. Those types of tools are invariably chucked in the workhead of a tool grinding machine having multiple drive mechanisms that impart compound motions to the workhead to move the tool cutting edge along appropriate contact with a grinding wheel. An exemplary machine for grinding cutting points on twist drills is shown in U.S. patent application Ser. No. 513,080, now U.S. Pat. No. 5,125,186.
The workhead chuck plays a very important part in the tool grinding process. The chuck must firmly and accurately grip the tool for grinding and release it after the grinding operation is completed. Many prior chucks include a chucking collet that is a permanent part of the workhead and a series of drill collets that interchangeably fit inside the chucking collet. The chucking collet and drill collet define a common longitudinal axis that is concentric with the axis of rotation of the workhead. The tool fits inside the drill collet. A drill collet can accommodate a small range of tool diameters.
Both the chucking collet and the drill collet are normally designed as a series of longitudinally extending ribs that are joined together on alternate ends. Some chucking collets, called single angle collets, have a single ring of material around the exterior of the ribs. The ring is formed with a frusto-conical surface. A piston that is part of the chuck has an internal frusto-conical surface that mates with the frusto-conical surface of the chucking collet. By moving the piston longitudinally along the workhead axis, the chucking collet is caused to collapse onto the drill collet, which also collapses to grip the tool inside the drill collet. A fixed surface is usually provided in the workhead to locate the chucking collet during the tool chucking actions Such that the chucking collet does not move the tool longitudinally during chucking.
Single angle collets have the disadvantage of being able to effectively collapse only under the ring with the frusto-conical surface. To provide stability to the tool about a transverse axis, a drill bushing is installed into and accurately located within the workhead concentric with the chucking and drill collets. One portion of the tool is accurately located within the drill bushing while another portion of the tool is clamped by the drill collet. An example of a single angle collet-drill bushing combination may be seen in FIG. 13 of U.S. Pat. No. 4,365,444.
Another drawback of the single angle collet-drill bushing design is that a different drill bushing is required for each diameter tool. Up to 250 drill bushings are required for standard inch and popular metric sized drills within the diameter range of 2.3 to 25.4 millimeters. Manufacturing multiple drill bushings and replacing worn drill bushings is expensive. Further, changing drill bushings with each change in diameter of the tool to be ground is a time consuming process. The combination single angle collet and drill bushing chucks also occupy an undesirably large volume of space.
To enable tools to be rigidly chucked without requiring a different drill bushing for each diameter tool, it is known to employ double angle collets. Such collets are generally similar to single angle collets, but they are usually longer and have two frusto-conical surfaces on their exteriors. The apexes of the two frusto-conical surfaces point away from each other. A pair of pistons within the workhead have frusto-conical surfaces that mate with the respective frusto-conical surfaces of the collet. Actuating the pistons longitudinally toward each other against the two collet frusto-conical surfaces collapses the double angle collet. A drill collet inside the double angle collet is also collapsed to grip a tool inside the drill collet. Because the two frusto-conical surfaces can be spread relatively far apart and thus grip a tool at relatively widely spaced locations, the double angle collet provides rotational rigidity to the tool without the use of drill bushings. An offsetting drawback, however, of prior double angle collets is that they cannot be longitudinally located against a reference surface. The double angle collet, together with the drill collet and the tool being gripped, move longitudinally during the chucking process a distance proportional to the amount of collapse required to grip the tool. After every chucking, therefore, the workhead and tool must undergo a preliminary search routine to reference the location of the tool point in space before grinding can begin. The tool referencing routine represents unproductive time.
Thus, a need exists for improvements in tool chucks.